It's not mAh. That's milliAmpere*hours, a measure of capacity. Chargers are rated in mA (milliAmperes).

It may not provide enough current to keep the N800 running (with much CPU/wireless usage) and charge at the same time. (Even the 890 mA charger (AC-4U) that comes with the N800 can sometimes do this, with full usage of the N800.) So they don't recommend it. Why not use the N800 charger for the other devices?